An optical transmitter (TX) module is an optical communications device used to transmit optical data signals over optical waveguides (e.g., optical fibers) of an optical communications network. An optical TX module includes input circuitry, a laser driver circuit, one or more laser diodes, and an optical coupling system. The input circuitry typically includes buffers and amplifiers for conditioning an input data signal, which is then provided to the laser driver circuit. The laser driver circuit receives the conditioned input data signal and produces electrical modulation and bias current signals, which are provided to the laser diodes to cause them to produce optical data signals having logic 1 and logic 0 intensity levels. The optical data signals are then directed by the optical coupling system onto the ends of respective transmit optical fibers held within a connector that mates with the optical transceiver module.
The optical TX typically also includes a closed loop optical output power feedback system that monitors and controls the modulation and/or bias currents of the laser diodes in such a way that the average optical output power levels of the laser diodes are maintained within designed range. In closed loop optical output power feedback systems, the optical coupling system of the TX module couples a portion of the light produced by the laser diodes onto respective monitor photodiodes in the TX module. The monitor photodiodes produce electrical signals corresponding to the optical output power levels of the laser diodes. Electrical feedback circuitry of the feedback system receives the electrical signals produced by the monitor photodiodes and produces control signals that are then used to adjust the modulation and/or bias currents of the laser diodes such that their average optical output power levels are maintained at designed levels.
Many optical coupling systems currently in use in optical TX modules incorporate relatively elaborate optical features for providing optical feedback, such as gratings and coated angled surfaces that diffract a portion of the light produced by the laser diodes to divert a portion of the light onto the optical feedback monitoring path. Manufacturing these types of optical features tends to be difficult and costly due to the complexity of the manufacturing processes.
A need exists for an optical coupling system for use in an optical TX module for optical feedback monitoring that can be manufactured at relatively low costs.